1. Field of the Invention
The present invention relates to a damage detection system using optical fiber sensors.
2. Description of Related Art
In a field which requires both strength and reduced weight for materials such as airframes of aircrafts or the like, it is essential that composite materials, e.g., carbon fiber reinforced plastics (CFRP), are widely applied in order to meet such requirements.
Damage detection apparatuses using FBG (Fiber Bragg grating)-type optical fiber sensors are disclosed in Patent Document 1 (Japanese Unexamined Patent Publication No. 2005-98921) and Patent Document 2 (U.S. Pat. No. 5,493,390, FIGS. 5A-5C), as apparatuses for detecting damages and defects occurred in such composite materials. The diameter of an optical fiber is becoming finer and finer (for example, diameter of 55 μm) in recent years. As a result, even when the optical fiber is embedded in a structure, the strength of the structure is not reduced so much. Therefore, an optical fiber has an advantage of a high degree of flexibility for its installation.
According to the technique described in Patent Document 1, the apparatus includes a piezoelectric element fixedly disposed on a predetermined position of a structural composite material, lead wires for transmitting a signal to the piezoelectric element, a optical fiber sensor fixedly disposed with a composite material for the structural composite material sandwiched between the optical fiber sensor and the piezoelectric element, and having a grating portion in a core thereof for reflecting light having a predetermined wavelength, a light source for irradiating light onto the core portion, and a characteristic detection unit to detect the characteristic of the light reflected from the grating portion. The piezoelectric element applies vibration to the material, and thereby damage is detected based on a change of the output from the characteristic detection unit. As for the characteristic detection unit, a spectrum analyzer is used for detecting frequency characteristics of the reflected light from the grating portion.
In the technique described in Patent Document 1, damage is determined depending on comparison with the detected data previously obtained from the healthy structural composite material. Alternatively, by setting a threshold value for a shift value of a specific frequency in a frequency distribution detected by the spectrum analyzer, the shift value being a value shifted from that obtained when no vibration is applied, it may be determined that damage exists when the shift value is not larger than the threshold value (paragraph 0032).
In the technique described in Patent Document 1, there is a need for a structure of a spectrum analyzer that can respond to a damage detection system with high accuracy.
On the other hand, the technique described in Patent Document 2 employs an opto-acoustical tuned filter as a spectrum analyzer.
In the technique described in Patent Document 2, a center wavelength of the reflected light, reflected out by the FBG optical fiber sensor, is fixed at center of the slope of the filter characteristic curve (FIGS. 5A and 5B), and an output value of the filter increases and decreases according to the vibration of the center wavelength of the reflected light when vibration is applied, which is represented by an output wave changing relative to the elapse of time (FIG. 5C) and obtained as basic data for detecting damage. In order to catch the change of the center wavelength of the reflected light, there is a need for such an FBG optical fiber sensor that has a sharp output characteristic of reflected light with a half-width (width at a half of the peak value) as small as possible.
On the other hand, it is required for highly accurate detection that a elastic wave of higher frequency has to be applied to a subject to be inspected, and the sensor length of the FBG optical fiber sensor is required to be short in order to detect the elastic wave of higher frequency with high sensitivity. However, the shorter sensor length causes the wave to be broader to make the half-width of the sensor output wider. The wider half-width of the sensor output causes the change of detection value to be smaller in the technique described in Patent Document 2. This causes a difficulty in capturing the wavelength change of the reflected light, so that a highly accurate damage detection system cannot be constructed.